Modular Design and Function

This code looks pretty decent. The program runs but it is incomplete. I need help making the program below work better. I did a MAIN function and the following funcations to compute the stress and strain in a steel rod of diameter D (inches) and length L (inches) subject to the compress loads P of 10,000 to 1,000,000 pounds in increments of 100,000 pounds. The modulus of elasticity E for steel is 30 x 10 to the 6th power.
Any help would be greatly appreciated.